1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic copying machine or a laser printer, and more particularly to an image forming apparatus which forms an image by use of a belt-like image forming member, in particular a belt-like photosensitive member and a belt-like intermediate transfer member.
2. Description of the Related Art
An image forming apparatus, e.g., an electrophotography copying machine or a printer, forms an image in such a manner that a toner image is formed on an electrostatic latent image carrier, e.g., a photosensitive drum, and is transferred onto a recording medium, e.g., a sheet of paper. Two methods of transferring the toner image (not yet fixed) to a recording medium are known. A first method directly transfers the toner image onto the recording medium. A second method primarily transfers the toner image onto an intermediate transfer member formed of a film member taking the form of a drum or an endless belt, and secondarily transfers the toner image from the intermediate transfer member onto a recording medium.
FIG. 9 is a diagram showing a structure of a color printer as an example of the image forming apparatus using a belt-like intermediate transfer member. The surface of a latent image carrier (hereinafter referred to as a photosensitive drum) 1 is uniformly charged with predetermined charges by a charger 2, and is subjected to a write scan process by use of a laser beam L, so that an electrostatic latent image is formed in accordance with a first-color image signal by writing and scanning of a laser beam L. With rotation of the photosensitive drum 1 in the direction A, the latent image reaches a position facing a first-color developing device of a developing unit 3, and is developed into a toner image T by the first-color developing device. The photosensitive drum 1 is further rotated while carrying the toner image T.
An intermediate transfer belt 4 moves at a speed substantially equal to the peripheral speed of the photosensitive drum 1, in synchronism with the toner developing operation. In a primary transfer portion where a primary transfer roll 5 is disposed in contact with the intermediate transfer belt 4 in the vicinity of a position right under a contact position where the photosensitive drum 1 comes in contact with the intermediate transfer belt 4, the toner image T is transferred from the photosensitive drum 1 to the intermediate transfer belt 4 under a transfer electric field, which is applied to the primary transfer roll 5 and has an electric polarity opposite to that of the toner. Here, a primary transfer cycle is completed.
The toner image that has been primarily transferred onto the intermediate transfer belt 4 reaches a secondary transfer portion where a secondary transfer roll 6 is disposed, with circulating motion of the intermediate transfer belt 4. In the case of a full-color image forming apparatus, the process from the latent image forming operation to the primary transferring operation is repeated for a preset number of colors (generally, yellow (Y), magenta (M), cyan (C), and black (Bk)) to form toner images of multiple colors on the intermediate transfer belt 4 in a superposed fashion.
To form those color toner images, the developing unit 3 consists of a rotary machine which is formed with an yellow developing device 3-1, a magenta developing device 3-2, a cyan developing device 3-3, and a black developing device 3-4. The developing unit 3 thus constructed is capable of developing latent images that have been formed on the photosensitive drum 1, in a successive manner.
The first-color toner image carried on the photosensitive drum 1 is thus transferred onto the intermediate transfer belt 4 at the primary transfer portion; residual toner is removed from the photosensitive drum 1 by a cleaner 7; the drum surface is electrically neutralized by a charge remover (not shown); and then another latent image corresponding to the second color is formed on the drum surface. The second-color latent image, like the first-color latent image, is developed in a similar manner, so that the second toner image is formed superposed on the first color toner image that has been previously transferred on the intermediate transfer belt 4. Third- and fourth-color latent images are similarly developed on the second color toner image on the intermediate transfer belt 4. In this way, those color toner images are superposed to form a multi-color toner image, not yet fixed, on the intermediate transfer belt 4.
At the instant that the intermediate transfer belt 4 having the multi-color toner image primarily transferred thereon reaches a secondary transfer position, a recording medium, or a sheet of recording paper P, having been fed from the paper tray 8, reaches the secondary transfer position.
When the sheet of recording paper P is transported while being nipped between the secondary transfer roll 6 and the intermediate transfer belt 4, the toner image is secondarily transferred from the intermediate transfer belt 4 onto the sheet of paper P under a transfer electric field that is applied to the secondary transfer roll 6 and has a polarity opposite to the charging polarity of the toner image.
The sheet of paper P bearing the multi-color toner image transferred thereonto is transported to a fixing unit (fuser) 9 which in turn heats and presses the toner image against the sheet of paper P to fix the multi-color toner image on the sheet of paper P. Here, an image forming process is completed. A charge remover (not shown) is disposed downstream of the secondary transfer roll 6 to remove charge from the sheet of paper P having undergone a secondary image transfer process.
The secondary transfer roll 6 is provided in a state that it may be brought into contact with and detached from the intermediate transfer belt 4 in the directions of arrows C. The roll 6 comes into contact with the intermediate transfer belt 4 when the sheet of paper P reaches the secondary transfer position, and is detached from the intermediate transfer belt 4 when the sheet of paper leaves there. The secondary transfer roll 6 returns to a stand-by position upon the end of the secondary transfer. A cleaner 10 disposed facing the intermediate transfer belt 4 is also brought into contact with the intermediate transfer belt 4 to clean the toner residual on (not transferred to) the belt 4, and detached therefrom after its removal.
Thus, in the color image forming apparatus using the intermediate transfer belt, the composite toner image (formed by superposing toner images) that has been already transferred onto the intermediate transfer belt in a superposing fashion is transferred onto the recording medium. Therefore, the apparatus is superior to the image forming apparatus of the type in which color toner images are successively and directly transferred onto the recording medium in that the composite image suffers from less misregistration and less deformation.
The intermediate transfer belt 4 is stretched by a drive roll 11, an idle roll 12, a secondary-transfer backup roll 13, and a tension roll 14, and driven by the drive roll 11 to move in the direction of arrow B. Widthwise-motion suppressing means including a rib and a rib guide is provided in association with the intermediate transfer belt 4. The suppressing means is for suppressing motions of the drive roll 11 and the like in the axial direction of the rolls.
The surface of the drive roll 11 is coated with high friction material so as to prevent slippage of the intermediate transfer belt 4 when the cleaner 10 and the secondary transfer roll 6 are loaded on the surface of the transfer belt 4.
Various proposals have been made to suppress a variation of a circulating velocity of the intermediate transfer belt 4, to render those rolls, e.g., the drive roll 11, immovable in their axial directions, to prevent the ends of the intermediate transfer belt 4 from being broken, and for other purposes.
Japanese Patent Unexamined Publication No. Hei. 2-27383 discloses a technique in which a rib is provided at one end (out of an image forming area) of the intermediate transfer belt, and grooves are provided in the rolls, while corresponding in position to the rib, and the coefficient of friction of the rib is different from that of the intermediate transfer belt. Japanese Patent Unexamined Publication No. Hei. 4-257888 discloses another technique in which ribs are formed at both ends of the belt put on the drive roll and the follower roll, and grooves are formed at both ends of the drive roll and the follower roll, while corresponding in position to the ribs.
Japanese Patent Unexamined Publication No. Hei. 5-134556 discloses a transfer belt with a tape (as a reinforcing member) stuck onto the end thereof. In this transfer belt, the outside diameter of the roll is reduced at its location corresponding to the reinforcing tape in order to prevent the transfer belt from rising at the contact portion of the roll and the tape and to prevent the boundary between the transfer belt and therein forcing member from being cracked.
Japanese Patent Unexamined Publication Nos. Hei. 9-175686 and Hei. 9-16512 disclose a technique in which the intermediate transfer belt is fastened to the rib by stitching, thereby preventing the intermediate transfer belt from slipping off the rib.
Japanese Patent Unexamined Publication No. Hei. 6-35331 discloses a technique for preventing the intermediate transfer belt from slipping on the drive roll. In this technique, irregularities of 20 to 100 .mu.m high are formed on the surface of the drive roll. Japanese Patent Unexamined Publication No. Hei. 8-152812 discloses a technique in which the inner surface of the intermediate transfer belt and/or the surface of the drive roll is coated with adhesive or high friction resin.
In the image forming apparatus which is provided with the intermediate transfer belt and the drive roll for driving it, and the combination of the rib and the rib guide for preventing the zig-zag motions of the belt and roll in their axial directions, the intermediate transfer belt is a semiconductive film, 50 to 100 .mu.m thick, consisting of a resin base made of polycarbonate or polyimide and resistance adjusting material. The surface of the drive roll is generally processed for high friction for preventing a slippage of the roll and the belt.
For the high friction process, the surface of the aluminum roll is coated with high friction resin, e.g., urethane rubber, so as to maintain a satisfactory coefficient of friction of the drive roll to the intermediate transfer belt for a long time. When the drive roll and the belt are new, the friction coefficient of the surface of the drive roll is too high. The result is that the belt repeats a stick slip in the axial direction to possibly squeak.
During the circulation of the intermediate transfer belt stretched out on a plural number of rolls, the belt takes a motion in its axial direction (the motion is called a walk). The walk is controlled to be within a predetermined amount of walk by the combination of the rib and the rib guide. When a state that the walk takes place and the rib and the rib guide mutually push continues for a long time, the end of the intermediate transfer belt will be broken in particular when the mechanical strength of the belt end is insufficient. In this state, the apparatus cannot continue its image forming operation.
Such a strong force as to break the intermediate transfer belt is caused by degradation of the flatness of the belt system, which is due to poor levelness of the apparatus body, twists caused by the stacking of component parts on the front and rear side plates of the apparatus body and assembling errors, circumferential length difference between both sides of the ends of the belt in the axial direction, and the like. There is a possibility that the intermediate transfer belt as the image carrier in the image forming apparatus can be broken to the intermediate transfer belt.
A mechanism to break the intermediate transfer belt will be described. The combination of a new drive roll and a new intermediate transfer belt has a high coefficient of friction, and hence a high gripping force is also created. Therefore, when the rolls supporting the intermediate transfer belt lose their alignment (parallelism of the axes of the rolls), the belt is liable to walk even if the misalignment is slight. In this case, the moving belt shifts sideways for a short time or after it has traveled several tens of cycles, and the rib abuts against the rib guide by a strong gripping force.
At a position where the belt is put on the drive roll and at a position where the belt leaves the drive roll, the following forces act on the side face of the rib. FIG. 10 is a cross sectional view showing the intermediate transfer belt 4 put on the drive roll 11, and FIG. 11 is a cross sectional view showing a contact state of the drive roll 11 with the intermediate transfer belt 4. In those figures, to prevent the walk of the intermediate transfer belt 4, ribs 41 are provided on both sides of the back surface of the intermediate transfer belt 4 in a state that it is in contact with the side faces of the drive roll 11.
When the intermediate transfer belt 4 walks and comes in contact with the side face of the drive roll 11, a force F1 acts on the side face of the rib 41 in a region R1 in which the intermediate transfer belt 4 begins to contact with the drive roll 11. The force F1 acts so as to cause the intermediate transfer belt 4 to rise and run onto the drive roll 11. In a region R2 where the lifted intermediate transfer belt 4 leaves the drive roll 11, the lift of the intermediate transfer belt 4 disappears.
FIG. 12 is a cross sectional view showing the intermediate transfer belt 4 when it is lifted. The intermediate transfer belt 4 is lifted by the force F1, while at the same time a strong pushing force acts on the side face of the rib 41, whereby a rise portion RU is formed. This rise portion RU disappears in the region R2. In the vicinity of the drive roll 11, the side ends of the intermediate transfer belt 4 are repetitively deformed alternately in one direction and the other direction that is opposite to the former: the side ends of the belt are repetitively subjected to an alternate process of the concentration and release of stress.
The force to press the rib 41 against the side face of the drive roll 11 increases as the gripping force is larger and the degree of misalignment is greater. In this state, the intermediate transfer belt 4 is liable to rise. When the rib 41 is forcibly pressed against the side face of the drive roll 11, the alternate concentration and release of stress is repeated and further the belt drive force is transmitted from the roll side face through the rib to the belt. The drive force to drive the belt is somewhat different from the drive force applied to the belt from the drive roll surface. The drive force difference produces a strain in the belt. The strain leads to accumulation of stress and generation of a squeaking sound by rubbing of the intermediate transfer belt 4 with the drive roll 11.
The concentration and release of stress are alternately repeated in the rise portion RU and the strain of the belt end is accumulated. When the operation of the image forming apparatus continues in this state, a fatigue is accumulated in the rise portion RU to give rise to a crack CR. The local crack CR grows into a breakage of the whole intermediate transfer belt 4 (FIG. 13). Further, there is a danger that a notch N of the end of the intermediate transfer belt 4 easily grows into the breakage of the whole intermediate transfer belt 4 (FIG. 14).
To prevent the walk of the intermediate transfer belt, it is, as a matter of course, necessary to secure accurate working of component parts and assembling of them. To this end, it is required that the rolls supporting the intermediate transfer belt are exactly aligned to one another and the intermediate transfer belt is accurately worked to have little difference of its circumferential length between the sides of the belt.
The approach of improving the mechanical precision of the intermediate transfer belt and its related rolls brings about the complexity of the steps of working, assembling and adjusting. In this respect, the approach is not suitable for the mass production of the image forming apparatuses. Even if the problems in the manufacturing stage are solved, the following problem is still present; when the image forming apparatus is installed on a place of poor levelness, it is impossible to secure the required accuracy of the alignment among the rolls that support the intermediate transfer belt.
To prevent the walk problem, it is necessary to strictly manage the precision of the component parts and assembling of them as described above. Further, some measure for improvement must be taken for other factors that may cause the walk producing the strong pushing force, e.g., the gripping force.
Incidentally, the image forming apparatus in which the intermediate transfer belt is controlled in its position by bringing the rib into contact with the ends of the rolls supporting the transfer belt, is disclosed in Japanese Patent Unexamined Publication No. Hei. 5-134556, already referred to. In the apparatus, a tape as the reinforcing member is applied to the end of the transfer belt for the purpose of preventing the belt end to be pressed against the roll ends from being deformed.
The above image forming apparatus composes the toner images of different colors on the intermediate transfer belt. Therefore, it is essential to accurately register those color toner images or to prevent a misregistration of those color toner images (referred to frequently as a color misregistration). To this end, it is necessary to accurately detect the reference position on the intermediate transfer belt and to control the operations of the related portions in the image forming apparatus in accordance with the detecting signal indicative of the reference position. To detect the reference position, the conventional technique detects a paint or a tape on the intermediate transfer belt, reads a mark (e.g., a through-hole) on the belt by use of a reflection type sensor, or reads a rotation position on the drive roll for the belt by use of an encoder.
In the reference-position detecting method using the mark of the through-hole, stress concentrates at the through-hole, possibly cracking the intermediate transfer belt. In the detecting method of reading the rotation position of the drive roll, an error that arises from slippage between the belt and the drive roll is liable to occur. The detecting method using the paint or tape is free from such problems.
However, the method using the paint or tape has the following problem. To reduce the misregistration of the color toner images, it is necessary to detect the mark of the paint or tape considerably accurately. For example, to reduce the color misregistration to 125 .mu.m, the mark detection error should be within 15 .mu.m.
To satisfy such a strict requirement, it is necessary to eliminate various factors causing detection errors, such as the traveling speed of the intermediate transfer belt, the bending and vibration of the intermediate transfer belt during its traveling, and the mounting position of the reflection type sensor.